1. Field of the Invention
The present invention relates to a new improved thermal analysis apparatus for analyzing changes of physical properties of a sample as a function of temperature, wherein the effect of the humidity of the periphery of a sample on the physical properties of the sample can be evaluated.
2. Background Information
Thermal analysis is an effective means for investigating how the physical properties of a material change with temperature. Examples of thermal analyzers include Differential Thermal Analyzers (DTA), Differential Scanning Calorimeters (DSC), Thermogravimetric Analyzers (TG), Thermomechanical Analyzers (TMA), and Dynamic Mechanical Analyzers (DMA). An improved thermal analysis apparatus providing a humidity control function is disclosed in Japanese Patent Laid-open No. Tokkaihei 8-145918.
Furthermore, an automatic moisture absorption equilibrium measurement device comprised of a combined microbalance and humidification device is available on the market.
In the prior art thermal analysis apparatus with a humidity control function described in Patent Laid-open No. Tokkaihei 8-145918, the program control is performed not only for the temperature but also for the humidity in the periphery of the sample. The humidity scanning measurement enabled by performing program control for humidity provides a great advantage. On the other hand, compared to the case of temperature equilibrium, the time required for the physical property of the sample at a certain humidity to reach equilibrium is extraordinary long. There is the problem that evaluation of the humidity dependency of the physical property of the sample is inaccurate unless the humidity scanning speed is set very low. Moreover, since the time required for the sample to reach humidity equilibrium depends on the shape and physical and chemical properties of the sample, there is the problem that the humidity program cannot be built adequately without obtaining knowledge about the hygroscopicity of the sample in advance.
On the other hand, an automatic moisture absorption equilibrium measurement device obtains desired relative humidity by mixing dry nitrogen and saturated vapor at a certain ratio, and comprises a precision gas flow rate control device, a vapor generating device, a mixing device, a drainage device, and a microbalance. The disadvantages are that this prior art device is expensive because of its complicated construction and lacks versatility as it cannot be used with a thermal analysis apparatus.
To measure the sample absorbed moisture at a certain humidity and reach the equilibrium state with the thermal analysis apparatus of the related art, during sampling and introduction of the sample, the device is exposed to an atmosphere in which the humidity is not controlled. Therefore, there is a problem that the system deviates from the equilibrium state before the measurement starts.
To resolve the above problems, an automatic humidity step control thermal analysis apparatus according to the present invention has a water chamber capable of temperature step control for generating saturated vapor at an appropriate temperature, a sample chamber for housing a sample and capable of changing the temperature and humidity of the sample, a heat insulation pipe for directing vapor from the water chamber to the sample chamber while preventing dew condensation, a detector for detecting and measuring physical properties of the sample, an a signal stability determination circuit for generating a trigger signal in case that the rate of change of the physical property signal from the detector drops below a specified reference value. In this case, the humidity of the sample chamber is determined according to the ratio of the saturated vapor pressures at the temperature of the hot water chamber and at the temperature of the sample chamber. The water temperature of the hot water chamber is changed corresponding to the trigger signal in a stepwise manner and, accordingly, the humidity of the sample chamber is also changed in a stepwise manner. As a result, the humidity equilibrium of the sample and the atmosphere in the periphery of the sample is confirmed based on the physical property of the sample, and a humidity condition is then automatically scanned in a stepwise manner.
Furthermore, if the sample, which previously absorbed moisture at a certain humidity and reached the equilibrium state, is caused to deviate from its state of equilibrium temporarily, the signal stability determination circuit determines the change of the physical property signal by controlling the humidity of the sample chamber and confirms whether or not it returns to its state of equilibrium with the trigger signal at the measurement start.